UNITED NATIONS — The U.N. nuclear chief said Monday that Iran is not cooperating with an investigation into suspected secret work on nuclear weapons.

Yukio Amano told the U.N. General Assembly that talks between the Vienna-based International Atomic Energy Agency and Iran have intensified this year after an IAEA report in November 2011 said it had "credible information that Iran had carried out activities relevant to the development of a nuclear explosive device," he said.

"However, no concrete results have been achieved so far," Amano said.

While the IAEA continues to verify that Iran's declared nuclear material is not being diverted from peaceful purposes, "Iran is not providing the necessary cooperation to enable us to provide credible assurance about the absence of undeclared nuclear material and activities," Amano said.

"Therefore, we cannot conclude that all nuclear material in Iran is in peaceful activities," he said.

But the IAEA director general said "the agency is firmly committed to intensifying dialogue with Iran."

"We will continue negotiations with Iran on a structured approach," he said. "I hope we can reach agreement without further delay."

Iran has repeatedly denied any interest in possessing nuclear arms, but the international community fears that Tehran may turn its peaceful uranium enrichment program toward weapons making — a concern that is growing as the government expands the number of machines it uses to enrich its stockpile of enriched uranium.

As those fears grow, so does concern that Israel could carry out its threats to attack Iran's nuclear facilities before that nation reaches the bomb-making threshold.

Iran's U.N. Ambassador Mohammad Khazaee reiterated his country's position, that it has a right to develop nuclear energy for peaceful purposes and rejected the claims in the IAEA report saying they are "not credible" and based on "forged reports" provided by Israel and the United States.

In his annual report to the world body, Amano said he also remains "seriously concerned" about North Korea's nuclear program, calling its statements about uranium enrichment activities and the construction of a light water reactor "deeply troubling."

In late 2010, Pyongyang unveiled a uranium enrichment facility that could give North Korea a second route to manufacture nuclear weapons in addition to its plutonium-based program. Earlier this year, satellite images showed that North Korea has made progress in building a light-water reactor to expand its nuclear program.

North Korea is under tough U.N. sanctions, and Amano called on Pyongyang to comply with its obligations under Security Council resolutions and the Nuclear Nonproliferation Treaty, and "to cooperate promptly and fully" with the IAEA.

North Korea's deputy U.N. ambassador Ri Tong Il rejected Amano's report, saying the country is not a party to the treaty, known as the NPT, or a member of the IAEA because the agency is not impartial and has "blindly" sided with the United States.

"Now, if you look at the current situation and developments on the Korean peninsula, the United States did not hesitate to escalate, aggravate, increase its threats and blackmails with increased hostilities toward the DPRK," he said, using the initials of the country's official name, the Democratic People's Republic of Korea.

"And the situation on the peninsula is on the brink of explosion, and nobody knows when the war will break out," Ri warned.

Under this situation, he said, six-party talks on the North's nuclear disarmament, which Pyongyang walked away from in 2009, have come "to the point of becoming almost a dead body at the moment." The talks include the U.S., China, Japan, Russia, North and South Korea.

Ri said North Korea is a full-fledged nuclear weapon state and should be treated the same way as other nuclear weapon states like the United States whose activities are not investigated by the IAEA.

IAEA inspectors were most recently expelled three years ago after the North quit the six-party talks. North Korea restarted its nuclear facilities and less than a month later, in May 2009, it conducted its second nuclear weapons test.

Amano also urged the Syrian government to respond to questions about a building destroyed by Israeli warplanes at the Deir El-zour site in the Syrian desert in 2007. The IAEA has said the building was "very likely" the covert site of a nuclear reactor.

The United States asserted more than four years ago that the bombed target was a nuclear reactor, but Syria has repeatedly denied allegations of any covert nuclear activity or interest in developing nuclear arms, saying the building was a non-nuclear military site.

Amano also reported on prospects for convening a conference in 2012 on the establishment of a Middle East zone free of nuclear weapons, which the 189 nations that are parties to the NPT called for in May 2010.

The IAEA held a forum on the possible relevance of creating such a zone in November 2011, he said, but "there remain fundamental differences of views among countries of the region on this important issue and it has not been possible to make further progress."

Egypt's U.N. Ambassador Mootaz Ahmadein Khalil stressed the importance of holding the conference in 2012 and told the General Assembly that any delay "will affect the effectiveness of verification activities in the Middle East" and "the integrity of international efforts to prevent nuclear proliferation."

He warned that it could even lead to the failure of the next conference to review the NPT in 2015.

Israel is not a party to the NPT and has long said a full Arab-Israeli peace must precede such weapons bans.

Thomas Mayr-Harting, head of the European Union delegation to the United Nations, said it is holding a seminar in Brussels on Monday and Tuesday where "an open exchange of views on all aspects related to the creation" of a nuclear weapons-free zone in the Mideast will take place.